The accurate measurement of surface temperatures of hot objects is of concern in many industrial and scientific processes. For instance, temperatures must be accurately measured during the processing and melting of metals and glasses, during the heat treatment of semiconductors, and during the fabrication of circuit chips. In particular, the accurate determination of the temperature of semiconductor wafers is especially needed during rapid thermal processing of the wafers, during rapid thermal oxidation, or during other processes which modify or add thin chemical films or coatings to the surface of the wafers. For these critical semiconductor fabrication processes, it is essential that the temperature be known within a few degrees over a range which may extend from less than 400.degree. C. to over 1,100.degree. C.
In the past, the temperature of hot objects was determined either using (1) contact methods or (2) non-contact methods. For instance, during contact methods, the hot object is contacted with a sensor such as a thermocouple that is in turn connected to a temperature meter, which indicates the temperature of the object. Conventional non-contact methods of determining temperature, on the other hand, include using a light sensor such as an optical pyrometer that senses the thermal radiation being emitted by the object at a particular wavelength of light. Once the thermal radiation being emitted by the object is known, the temperature of the object can be estimated.
When processing semiconductor materials for use in the electronics industry, it is far preferable to use non-contact methods when measuring the temperature of semiconductor wafers. For instance, one advantage of non-contact methods is that the wafer can be spun slowly during the heating process so that the temperature is more uniform over the surface of the wafer. Rotating the wafer also promotes more uniform contact between the flow of processing gases and the wafer. Besides being able to rotate the wafers, another advantage to using non-contact methods is that, since no temperature gauges need be attached to the wafer, the wafers can be processed much more quickly saving precious time during semiconductor fabrication.
For all of the high temperature wafer processes of current and foreseeable interest, one of the more important requirements is that the true temperature of the wafer be determined with high accuracy, repeatability and speed. The ability to accurately measure the temperature of a wafer has a direct payoff in the quality and size of the manufactured semiconductor devices. For instance, the smallest feature size required for a given semiconductor device limits the computing speed of the finished microchip. The feature size in turn is linked to the ability to measure and control the temperature of the device during processing. Thus, there is increasing pressure within the semiconductor industry to develop more accurate temperature measurement and control systems.
In this regard, the chief disadvantage of conventional non-contact optical pyrometry systems for determining temperature is that the systems measure an apparent temperature rather than the true temperature of the wafer. In particular, a real surface emits radiation less efficiently than an ideal or perfect blackbody. Through theory and calculation, once the emitted radiation of a blackbody is known, the temperature of the blackbody can be calculated. A real body, however, such as a wafer, emits only a fraction of the radiation that would be emitted by a blackbody at the same temperature. This fraction is defined as the emissivity of the real object. Thus, when sensing the radiation being emitted by a real body, a pyrometer generally indicates an apparent temperature that is lower than the true temperature of the object.
Thus, in order to measure the true temperature of a real body using a pyrometer, the indicated temperature must be corrected to account for the emissivity. Unfortunately, the emissivity of a real body is generally unknown and is very difficult to measure accurately. Further, the emissivity of semiconductor wafers varies from wafer to wafer. The emissivity is a property of the surface and depends on several parameters, such as the chemical composition of the wafer, the thickness of the wafer, the surface roughness of the wafer, and the wavelength at which the pyrometer operates.
In the past, others have attempted to approximate the emissivity of a semiconductor wafer or to otherwise minimize its impact on temperature measurements using a pyrometer. For instance, one method for approximating the temperature of a silicon wafer using a pyrometer is to first determine the emissivity of the wafer or of a similarly constructed wafer using a temperature thermocouple in a separate process. This method, however, is not efficient. Further, it has been found that the emissivity of silicon wafers, even if they are similarly constructed, can vary widely from wafer to wafer.
Besides attempting to determine the emissivity of a wafer, other methods attempt to diminish the effect of not knowing the emissivity by using emissivity enhancement techniques. During these techniques, the object is to artificially increase the emissivity of the wafer to a value very close to unity which causes the wafer to simulate a blackbody allowing for more accurate temperature readings. For instance, one known emissity enhancement technique is to place a highly reflective sheet parallel to the semiconductor wafer as disclosed in a published European Patent Application having Publication No. 0612862 entitled: "Measuring Wafer Temperatures" by Gronet et al. and in U.S. Pat. No. 5,226,732 to Nakos et al. which are both incorporated herein by reference.
By placing a reflective sheet next to the wafer, the radiation emitted by the wafer reflects multiple times. The multiple reflections between the wafer and the reflective sheet cause the radiation between the two surfaces to add up and approximate the radiation of a perfect blackbody at the temperature of the wafer. This has the effect of enhancing the emissivity of the wafer to a value close to unity, allowing for more accurate temperature measurements.
For instance, according to Planck's law the radiation emitted by a blackbody at a particular wavelength (.lambda.) and temperature (T) is as follows: ##EQU1## wherein C.sub.1 and C.sub.2 are well known constants. The radiation emitted by a real surface, such as a wafer, with an emissivity (E), on the other hand, is as follows: EQU R.sub.w =ER.sub.bb ( 2)
When a reflective sheet is placed adjacent to the wafer, at a distance between the reflective sheet and the wafer which is vanishingly small, an infinite series of rays are created which have increasing numbers of reflections. A pyrometer sensing the multiple reflections would measure a total radiation emitted by the wafer as follows: EQU R.sub.w =ER.sub.bb 1+.rho..sub.r .rho..sub.w +(.rho..sub.r .rho..sub.w).sup.2 . . . ! (3)
wherein .rho..sub.r is the reflectivity of the reflective sheet and .rho..sub.w is the reflectivity of the wafer.
Because the geometric series contained in the brackets above simplifies to 1/(1-.rho..sub.r .rho..sub.w) and because, according to Kirchhoff's law E=1-.rho..sub.w, the radiation emitted by the wafer can be stated as follows: ##EQU2##
Finally, if the reflectivity of the reflective sheet (.rho..sub.r) is nearly 100%, i.e., 1, then the above equation reduces to EQU R.sub.w =R.sub.bb ( 5)
However, if the distance between the wafer and the reflective sheet is increased to several millimeters, as can be necessary in a practical system, the value of the enhanced radiation measured by the pyrometer as described above, is still dependent on the emissivity of the wafer's surface and on the reflectivity of the reflective sheet. Thus, better results are achieved when the starting emissivity of the wafer is already high. In particular, the above methods are not reliable when the wafer's emissivity is low, such as in the range of from about 0 to about 0.3. Consequently, the implementation of known emissivity enhancing techniques as described above have good but limited payoffs. This in turn restricts the accuracy with which the true temperature of the wafer can be determined, which in turn limits the performance and accuracy of the wafer processing system.